Circuit arrangement for regulating the speed of a dc motor

ABSTRACT

A circuit arrangement for controlling the speed of a DC motor by comparing a first voltage, which varies with the speed of the motor, with a second, control voltage, and varying the current fed to the motor in accordance with the resultant of the two voltages. A bridge circuit including a first current amplifier supplies a constant control voltage (the second voltage); the control voltage is compared with the first voltage by a differential amplifier. The resultant voltage is fed to a second current amplifier the output of which controls current input to the motor. The first voltage is preferably supplied by a generator driven in synchronism with the motor, and the second voltage is preferably supplied by a bridge having a semiconductor in one branch thereof. The circuit includes temperature-dependent resistances which compensate for temperature variations in the generator, as well as in the circuit arrangement generally.

United States Patent [191 m1 3,777,234 Luger Dec. 4, 1973 CIRCUITARRANGEMENT FOR REGULATING THE SPEED OF A no MOTOR Inventor: AugustLuger, Munich, Germany Assignee: Agfa-Gevaert Aktiengesellschaft,

' Leverkusen, Germany Filed: Sept. 13, 1971 Appl. No.: 180,008

Related US. Application Data Continuation of Ser. No. 889,632, Dec, 31,1969, abandoned.

Primary ExaminerBernard A. Gilheany Assistant Examiner-Thomas LangerAtt0mey-Arthur O. Klein [57 ABSTRACT A circuit arrangement forcontrolling the speed of a DC motor by comparing a first voltage, whichvaries with the speed of the motor, with a second, control voltage, andvarying the current fed to the motor in accordance with the resultant ofthe two voltages. A bridge circuit including a first current amplifiersupplies a constant control voltage (the second voltage); the controlvoltage is compared with the first voltage by a differential amplifier.The resultant voltage is fed to a second current amplifier the output ofwhich controls current input to the motor. The first voltage ispreferably supplied by a generator driven in synchronism with the motor,and the second voltage is preferably supplied by a bridge having asemiconductor in one branch thereof. The circuit includestemperature-dependent resistances which compensate for temperaturevariations in the generator, as well as in the circuit arrangementgenerally.

11 Claims, 1 Drawing Figure CIRCUIT ARRANGEMENT FOR REGULATING THE SPEEDOF A DC MOTOR This is a continuation, of application, Ser. No. 889,632filed Dec. 31, 1969, now abandoned.

This invention relates to a circuit arrangement for adjusting the speedof rotation of a direct current motor, and more particularly to acircuit arrangement for maintaining the motor speed at a predetermineddesired value. The circuit arrangement of the invention utilizes acontrol voltage which is compared to a control voltage dependent on thespeed of the DC motor. Direct current motors provided with speedcontrolling means in accordance with the invention may be used, forexample, in microfilm development apparatus, photocopy machines andinstallations having dosing or injecting pumps, in which the maintenanceof a constant speed of rotation of the motor is very important.

A known circuit arrangement for controlling the speed of a DC motoremploys a control voltage, which is produced by a known circuitarrangement including a zener diode and a voltage divider. The output ofthe voltage divider is applied to a first end of a resistance the secondend of which is connected to the input of an amplifier. A. furthervoltage is superimposed on the control voltage at the second end of theresistance which is in series with said control voltage, said furthervoltage being opposite to the control voltage. This further voltage isproduced by a generator which is connected to the shaft of the motor.The voltage composed of the aforementioned two voltages is imposed onsaid amplifier stage, by means of which output transistors forming partof the motor circuit are adjusted or controlled. This known type ofcircuit arrangement has the disadvantage that, when the operationvoltage of the motor supply varies, as well as when temperature changesoccur, the rotational speed 'of the motor does not remain constant.

In accordance with the circuit arrangement of this invention, it ispossible, however, to obtain with a predetermined control voltage, aconstant motor speed which is independent of variations of the motorpower supply voltage and of temperature changes in the circuitarrangement and in the generator. Furthermore, the circuit arrangementof this invention is less sensitive to load changes than'are previouslyknown circuit arrangements.

The objects of the invention can be achieved with relatively simplecircuit control means. A preferred circuit arrangement in accordancewith the invention includes a bridge circuit which is energized by adirect current supply voltage, one branch of the bridge having at leastone semi-conductor element which supplies a substantially constantvoltage; in a further bridge branch there is provided an adjustableresistance. The bridge output is connected to a current amplifier stageat the output of which an adjustable control voltage is taken off. Thecurrent amplifier stage is connected to a differential amplifier whichserves as a comparator between the control voltage and the voltageapplied from the motor-driven generator. The output of the differentialamplifier is connected with a further current amplifier stage by meansof which the motor current of the DC motor is maintained constant sothat the predetermined speed of rotation of the motor can be adjusted inaccordance with the control voltage. That part of the input circuit ofthe differential amplifier which is mounted in the generator has atleast two temperature-dependent resistances which have a definiterelationship with regard to the temperature, such resistances serving tocompensate for temperature variations within the generator as well as inthe circuit arrangement generally.

Preferably the semi-conductor element employed in the control voltagesupplying bridge is at least one zener diode, and the resistance locatedon the adjacent bridge branch is adjustable in such a way that thevoltage changes which manifest themselves in the zener diode arecompensated by the resistance at the bridge output.

Since the zener diodes have finite internal resistances, a constantcontrol voltage cannot be produced when the supply voltage varies.Furthermore, the input resistance of the current amplifier which isconnected to the output of the bridge circuit is so large that thecurrent flowing through it is negligibly small in comparison to thecurrent flowing to the bridge branches. The current variations whichoccur in the zener diode can thus be compensated at the bridge output bymeans of an adjustable compensating resistance in the adjacent bridgebranch.

In order for the input resistance of the current amplifier, which is fedby the bridge output, to be high, the current amplifier has at least onetransistor connected in emitter-follower relationship; the outputcircuit of the bridge has at least one adjustable resistance interposedtherein. By means of this adjustable resistance, the control voltage canbe adjusted in accordance with the selected speed. of the DC motor. Theemitterfollower, which is connected to act as a current amplifier, alsofunctions as an impedance converter. Current variations which occur inthe emitter circuit are hardly noticeable at the input, that is, thereis little feedback.

In accordance with the disclosed embodiment of the invention, there ispresent in the emitter circuit of the current amplifier a multi-stagevoltage divider by means of which a desired voltage may be selected bymeans of a multi-position switch. The illustrative differentialamplifier has two transistors, the emitters of which are connected to acommon resistance. The collector resistance of the first transistor islow-ohmic and the collector resistance of the second transistor ishighohmic. The first transistor is controlled by the control voltage andthe second transistor is controlled by the generator voltage. The firsttransistor of the differential amplifier also acts as animpedance-converter in order to put still less of a load on the bridgecircuit. Since the collector resistance of the second transistor ishighohmic, the current flowing through the second transistor does notsubstantially affect the control voltage drop at the common emitterresistance. Due to this, a voltage increase or a voltage decrease causedby this current at the emitter resistance is negligibly small, so thatthe voltage at the emitter resistance can be assumed, with an adjustedcontrol voltage, to be almost constant.

The additional current amplifier preferably includes transistorsconnected in cascade. By means of this further current amplifier, thehigh-ohmic output resistance of the differential amplifier is adjustedto an optimum degree to that of the small resistance of the DC motor, sothat the small currents of the output circuit of the differentialamplifier can be amplified in a sufficient manner for the direct currentmotor.

In order to avoid damage to or destruction of the DC motor when it isstalled, there is provided a currentlimiting means in the circuit whichconnects the differential amplifier with the additional currentamplifier. By means of this limiting arrangement the additional currentamplifier can be controlled.

According to a preferred embodiment of the invention, thecurrent-limiting arrangement has a transistor, the collector of which isconnected to the base of the input transistor of the additional currentamplifier, the emitter of which is connected to the emitter of theoutput transistor of the additional current amplifier in the base ofwhich is connected through a potentiometer with the base and emitter ofthe output transistor which are connected by the total resistance of thepotentiometer.

In order to compensate for temperature variations in the generator aswell as in the circuit generally, there is provided, in the region ofthe differential amplifier, a temperature-dependent resistance in thebase branch of the transistor which is connected with the generator, andin series with this resistance in the region of the generator there isprovided a further temperaturedependent resistance. Thetemperature-dependent resistances have the characteristic of decreasingtheir resistances as temperature increases.

Advantageously there may be used, as temperaturedependent resistances,resistances of the NTC types. It

is furthermore possible to provide an ohmic resistance in parallel withthe temperature-dependent resistances at all times.

The invention is illustrated in conjunction with a single FIGURE whichis a circuit diagram of a preferred embodiment of the invention.

In the drawing there is illustrated a DC motorwhich is designated by theletter M and a generator which is designated with the letter G. Analternating current supply voltage 3 which is connected to terminals 1and 2 is rectified by means of a rectifier arrangement 4 which isprovided with a current-smoothing condenser 5. A bridge circuit isarranged parallel to the condenser 5; in the upper two branches of thebridge circuit there are resistances 6,7 and 8 arranged as shown. In afurther, lower branch of the bridge there are disposed two seriesconnected zener diodes 9 and 10. In the remaining, other lower branch ofthe bridge circuit there is disposed a further, adjustable resistance11. The resistances 6, 7, 8 and 11 are selected in such a way that theycompensate each other, so that when circuit voltage variations occur thevoltage variation at the zener diodes 9 and is compensated by thevoltage at the resistance 11, which is opposed to the voltage atthezener diodes 9 and 10. The diagonally opposed output terminals of thebridge thus delivera constant voltage. The bridge circuit is connectedto a first current amplifier having a transistor 12 with anemitter-follower which operates as an impedance converter under thecontrol of the voltage of the power source 49 for the motor M. Thecollector of transistor 12 is connected to a first main wire A of thecircuit through a resistance 32. In the emitter circuit of thetransistor 12 there is provided a multi-stage voltage divider betweenthe series connected resistances of which 13 to 21, incl., tap contacts22 to 29, incl., are provided. The lower end of resistance 21 isconnected to a second main wire B of the circuit. Tap contacts 22 to 29,incl., are connected to a differential amplifier via a contact arm 31which current source, such as a rectified A. C. source 49. The

condenser 35 smooths out the voltage variations caused by travel of arm31 from one of contacts 22 to 29, incl., to another. The collector oftransistor 34 is connected to wire A through a resistance 36 of lowresistance value. Wire A is connected to the positive terminal 54 of theabove-mentioned current source 49. The differential amplifier has afurther transistor 37, the emitter of which is connected to the emitterof transistor 34; the junction between the two emitters is connected towire B through a resistance 38. The collector of transistor 37 isconnected to wire A through a resistance 39 of high resistance value.The base of transistor 37 is connected through two series connected NTCresistances 40, 41 to the upper terminal of the generator G. In parallelwith both temperature-dependent resistances 40, 41 there are providedresistances 42 and 43, respectively. The base of the transistor 37 isalso connected with a voltage divider resistance 44. There is provided acondenser 45 connected to wire B in the region of the generator G forsmoothing out the generator voltage. The-lower terminal of the generatoris connected to thenegative wire B.

The collector of the transistor 37 is connected with the base of theinput transistor 46 of the additional current amplifier stage which isshown as of the cascade type having three transistors. The emitter ofthe transistor 46 is connected with the base of a further transistor 47,the emitter of which is connected to the base of an output transistor48. The transistor 48 is preferably a high capacity power transistor.The collectors of the transistors 46, 47 and 48 are connected to thepositive wire A. The collector and emitter of the output transistor 48are connected in series with the upper terminal of the DC motor M. Thesecond terminal of the motor is connected to the negative wire B. Avoltage smooth- 1 ing condenser 50 is provided between the negative wireB and the emitter of the output transistor 48. A voltage divider orpotentiometer 51 is connected across the base-emitter of the transistor48. The take-off leg 52 of the potentiometer is connected to the base ofa transistor 53, which acts as a current limiter. The collector of thetransistor 53 is connected to the collector of the transistor 37, whichforms part of the differential amplifier, whereas the emitter of thetransistor 53 is connected to the emitter of the output transistor 48.

The circuit arrangement of this invention operates as follows:

It is assumed that, in the illustrated position of the circular switch31, the motor M operates at a stable, constant speed, and that thedescribed compensating elements in the circuit are functioning tomaintain such condition. The contact arm 31 and the contact point 24 ofthe voltage divider 13 21, incl., are connected with the base oftransistor 34. As a result of this, the base of the transistor 34 isprovided with a control voltage which adjusts the transistor 34 in sucha way that the flow of collector current of the transistor 34 throughthe resistance 38 brings about a constant voltage drop. Because of suchvoltage drop, the emitter of the transistor 37 has a correspondingincrease of its voltage value.

It is assumed that the motor M is subjected to a constant load. Thespeed of rotation of the motor has accordingly been adjusted to a valuecorresponding to the adjusted control voltage. Voltage produced by thegenerator G causes a voltage drop at the resistance 44. The voltagedifferential between this voltage and the control voltage at theresistance 38 controls the transistor 37 in such a way that a current ofa predetermined size flows through the collector resistance 39; suchcurrent flow causes a voltage drop in the resistance 39.The currentwhich flows through the base of the transistor 46 is now amplified bymeans of the further connected transistors 47 and 48 to such a valuethat, by means of this current, the speed of rotation of the motorremains constant. If, for example, the motor is more strongly loaded,the motor will tend to rotate at a slower speed. As a result of this,the voltage furnished by the generator G and thus the voltage at thebase of the transistor '37 is lowered by a corresponding amount. Thiscauses a reduction of current flowing through the resistance 39, so thatthe base of the transistor 46 becomes more positive. The base current ofthe transistor 46 thus increases. This increased current is amplified bythe further transistors 47, 48. The current flowing through the DC motorM is consequently increased by an amount which will immediatelycompensate thedrop in the rotational velocity of the motor. As the loadon the motor M increases, its speed of rotation is maintained constantby the described sensitive circuit arrangement.

The same is true when the load of the motor decreases. If, for example,the contact arm 31 rotates to make contact with the contact 26, therebyto reduce the control voltage at the resistance 38 with respect to thepreviously adjusted control voltage value, the voltage between the baseand emitter of the transistor 37' is increased by a correspondingamount. As a result of this, there is produced an increase of thecurrent flowing through the resistance 39 and a consequent throttlingof, or decrease in the current flow through, the transistors 46, 47 and48. As a result of this, the current flowing through the motor M isdecreased so that, with a given constant load of the motor M, its speedof rotation is increased..At the same time, the voltage drop at theresistance 44 of the generator G is also decreased. The current flowingthrough the resistance 39 is also reduced until the speed of rotation ofthe motor M reaches the value corresponding to that of the preselectedcontrol voltage.

If, for example, the motor M is held from rotation, the generatorvoltage sinks to zero, which causes the transistor 37 to be locked innon-conducting condition.

Since, in this case, the base of the transistor 46 has the positivepotential of thewire A, the transistors 46, 47 and-48 are in theirmaximum current-carrying condition. In order to protect the motor Magainst overload, the transistor 53 is now, to a very large degree,adjusted in accordance with the voltage drop at the voltage divider 51,so that a current flows via the resistance 39 and the collector-emitterpath of the transistor 53. This causes the base of the transistor 46 tobecome more negative and, consequently, the transistors 46,47 and 48 areadjusted or controlled to pass a smaller amount of current.

' 6 A thermal switch (not shown) may be provided to disconnect the motorM as well as the circuit arrangement generally from the voltage sourceupon a prolonged blocking of the motor. When, for example, the

contact arm 31 is swung to the contact point 22, the voltage drop whichoccurs at the resistance 38 is raised vis a vis the control voltageprovided when arm 31 engages contact 24. As a result of this, thetransistor 37 is controlled so as to cause increased current flowthrough the motor M so that the motor speed increases. As a consequence,the voltage furnished by the generator G is also increased. Theincreased increase of the current demand by the motor M is terminatedwhen the motor speed which corresponds to a selected control voltage hasbeen reached.

If temperature variations should occur at the generator, the inductionof the generator G is changed. It is assumed that a temperature increasecauses a decrease of such induction. This will cause a decrease of thegenerator voltage, so that the compensating voltage at the base of thetransistor 37 is also reduced. This causes an increase in the speed ofthe motor M. Since, however, with increasing temperature, the resistance41 of the generator G is reduced. The voltage at the base of thetransistor 37 is maintained constant. In other words, the reduction ofthe generator voltage is equalized by the decrease of the resistance.Consequently, the speed of the motor M remains constant despitetemperature variations which may occur in the generator.

If temperature variations occur in the circuit arrangement generally,the electrical characteristics of the semi-conductor elements, forexample, those of the transistors and zener diodes, are changed in sucha way that a change in motor speed would occur because of variations atthe zenerdiodes and transistors caused by the changes in temperature.The temperatureresistance 40, however, compensates for such changes.Resistance 40 is arranged in a circuit in the immediate vicinity of thezener diodes 9, 10 andthe transistors 12, 34, 37, 46, 47 and 48. Whenthe temperature increases, the resistance value of the resistance 40 isdecreased in such a way that the increase of the speed of the motor,which would normally occur due to a temperature increase, in the absenceof resistance 40 is compensated, in other words, motor speed remainsconstant.

It will be seen that the power source 49 for the motor M supplies thepower for the control circuit arrangement; the voltage of such source,which is applied to the collector of transistor 12 through resistance32, plays a significant role in the operation of the control circuit,since the collector-emitter current flow through transistor 12 varieswith changes in the voltage of. source 49. It is for this reason, amongothers pointed out above, that the circuit compensates for variations inthe voltage of the motor supply source 49 so as to maintain the speed ofthe motor M substantially constant.

Although my invention has been illustrated and described with referenceto one preferred embodiment thereof, I wish to have it understood thatit is in no way limited to the details of this one embodiment but iscapable of numerous modifications within the scope of the appendedclaims.

What is claimed is: p 1. Apparatus for regulating the speed of a DCmotor, which comprises:

means coupled to the motor for generating a first DC voltage normallyproportional to the motor speed, said generating means being susceptibleto temperature-dependent changes in the first DC voltage;

a differential amplifier having first and second inputs and an output,the amplifier being susceptible to temperature-induced changes in itsoperating point;

temperature-dependent resistive means interconnecting the output of thefirst DC voltage generating means and the first input of thedifferential amplifier, said resistive means being individuallyassociated with the last-mentioned generating means and with thedifferential amplifier for respectively compensating both thetemperature-induced changes in the first DC voltage and thetemperature-induced changes in the operating point of the differentialamplifier;

means responsive to the output of the differential amplifier forcoupling to the motor a second DC voltage proportional to the output ofthe differential amplifier, the motor speed being normally proportionalto the second DC voltage;

means for generating a stable motor speed control voltage comprising, incombination, an emitter follower having adjustable output resistancemeans, and means including a single resistance bridge circuit having aninput connectable to a third DC voltage and an output coupled to theinput of the emitter follower, the bridge circuit including a Zenerdiode in one arm and an adjustable resistance in another arm forcompensating the finite resistance of the Zener diode, the output of theemitter follower constituting the output of the control voltagegenerating means; and

means for coupling the output voltage of the control voltage generatingmeans to the second input of the differential amplifier.

2. Apparatus as defined in claim 1, in which the ad-, justable outputresistance means of the emitter follower comprises a plurality ofserially connected resistors defining a corresponding number of outputtaps, and in which the output voltage coupling means comprises switchingmeans for interconnecting a selected ones of the taps to the secondinput of the differential amplifier.

3. Apparatus as defined in claim 1, in which. the temperature-dependentresistance means comprises, in combination, first and second seriallyconnected temperature-dependent resistances, means for associating thefirst resistance with the generating means to expose the firstresistance to temperature changes of the generating means, and means forassociating the second resistance with the differential amplifier toexpose the second resistance to temperature changes of the differentialamplifier.

4. Apparatus as defined in claim 3, in which each of the first andsecond resistances has a negative temperature coefficient.

5. Apparatus as defined in claim 3, further comprising an additionalpair of resistors individually shunting the first and secondresistances.

6. Apparatus as defined in claim 1, in which the differential amplifiercomprises, in combination, first and second transistors connected incommon-emitter configuration, a common emitter resistor, a relativelyhigh resistance in the collector circuit in the first transistor, and arelatively low resistance in the collector circuit of the secondtransistor, the bases of the first and second transistors individuallyconstituting the first and second inputs of the differential amplifierand the collector of the first transistor constituting the output of thedifferential amplifier.

7. Apparatus as defined in claim 1, in which the second DC voltagecoupling means comprises third, fourth and fifth transistors connectedin cascade.

8. Apparatus as defined in claim 7, further comprising means coupled tothe collector of the first transistor for reducing the second DC voltagewhen the motor is over-loaded.

9. Apparatus as defined in claim 7, in which the base of the thirdtransistor is coupled to the output of the differential amplifier and inwhich the collector emitter-circuit of the fifth transistor is connectedin series with the DC-motor.

10. In a speed control system for a DC motor wherein the systemcomprises, in combination, a differential amplifier having first andsecond inputs, means responsive to the output of the differentialamplifier for coupling to the motor a first DC voltage proportional tothe output voltage of the differential amplifier, means coupled to themotor for generating a second DC voltage proportional to the motorspeed, means for generating a third DC voltage corresponding to adesired speed of the motor, means for coupling the second DC voltage tothe first input of the differential amplifier, means for coupling thethird DC voltage to the second input of the differential amplifier, andstabilizing means associated with the system for maintaining the speedof the motor relatively constant with changes in temperature, theimprovement wherein:

' the third DC voltage generating means comprises, in combination, anemitter follower having adjustable output resistance means, and meansincluding a 7 single resistance bridge circuit having an inputconnectable to a DC source and an output coupled to the input of theemitter follower, said bridge circuit including a Zener diode in one armand an adjustable resistance in another arm for compensating the finiteresistance of the Zener diode, the output of the emitter followerconstituting the output of the third DC voltage generating means; and

the stabilizing means comprises dependent resistive meansinterconnecting the output of the second DC voltage generating means andthe first input of the differential amplifier, said resistive meansbeing individually associated with the last-mentioned generating meansand the differential amplifier for respectively compensatingtemperature-induced changes in the second DC voltage andtemperature-induced changes in the operating point of thedifferentialamplifier.

11. Apparatus for regulating the speed of a DC motor, which comprises:

means coupled to the motor for generating a first DC voltage normallyproportional to the motor speed, said generating means being susceptibleto temperature-dependent changes in the first DC voltage;

a differential amplifier having first and second inputs and an output,the amplifier being susceptible to temperature-induced changes in itsoperating point;

temperature-dependent resistive means interconnecting the output of thefirst DC voltage generating means and the first input of thedifferential amplifier, said resistive means being individuallyassociated with the last-mentioned generating means and with thedifferential amplifier for respectively temperaturecompensating both thetemperature-induced changes in the first DC voltage and thetemperature-induced changes in the operating point of the differentialamplifier;

means responsive to the output of the differential amplifier forcoupling to the motor a second DC voltage proportional to the output ofthe differential amplifier, the second DC voltage coupling meanscomprising first, second and third transistors connected in cascade;

a voltage divider having input terminals connected between the outputand input of the third transistor;

means coupled to the output of the differential amplifier for reducingthe second DC voltage when the motor is overloaded, the reducing meanscomprising a fourth transistor having a collector connected to the inputof the first transistor, an emitter connected to the output of the thirdtransistor and a base connected to the output of the voltage divider;means for generating a stable motor speed control voltage comprising, incombination, an emitter follower having adjustable output resistancemeans, and means including a single resistance bridge circuit having aninput connectable to a third DC voltage and an output coupled to theinput of the emitdifferential amplifier.

1. Apparatus for regulating the speed of a DC motor, which comprises:means coupled to the motor for generating a first DC voltage normallyproportional to the motor speed, said generating means being susceptibleto temperature-dependent changes in the first DC voltage; a differentialamplifier having first and second inputs and an output, the amplifierbeing susceptible to temperature-induced changes in its operating point;temperature-dependent resistive means interconnecting the output of thefirst DC voltage generating means and the first input of thedifferential amplifier, said resistive means being individuallyassociated with the last-mentioned generating means and with thedifferential amplifier for respectively compensating both thetemperature-induced changes in the first DC voltage and thetemperature-induced changes in the operating point of the differentialamplifier; means responsive to the output of the differential amplifierfor coupling to the motor a second DC voltage proportional to the outputof the differential amplifier, the motor speed being normallyproportional to the second DC voltage; means for generating a stablemotor speed control voltage comprising, in combination, an emitterfollower having adjustable output resistance means, and means includinga single resistance bridge circuit having an input connectable to athird DC voltage and an output coupled to the input of the emitterfollower, the bridge circuit including a Zener diode in one arm and anadjustable resistance in another arm for compensating the finiteresistance of the Zener diode, the output of the emitter followerconStituting the output of the control voltage generating means; andmeans for coupling the output voltage of the control voltage generatingmeans to the second input of the differential amplifier.
 2. Apparatus asdefined in claim 1, in which the adjustable output resistance means ofthe emitter follower comprises a plurality of serially connectedresistors defining a corresponding number of output taps, and in whichthe output voltage coupling means comprises switching means forinterconnecting a selected ones of the taps to the second input of thedifferential amplifier.
 3. Apparatus as defined in claim 1, in which thetemperature-dependent resistance means comprises, in combination, firstand second serially connected temperature-dependent resistances, meansfor associating the first resistance with the generating means to exposethe first resistance to temperature changes of the generating means, andmeans for associating the second resistance with the differentialamplifier to expose the second resistance to temperature changes of thedifferential amplifier.
 4. Apparatus as defined in claim 3, in whicheach of the first and second resistances has a negative temperaturecoefficient.
 5. Apparatus as defined in claim 3, further comprising anadditional pair of resistors individually shunting the first and secondresistances.
 6. Apparatus as defined in claim 1, in which thedifferential amplifier comprises, in combination, first and secondtransistors connected in common-emitter configuration, a common emitterresistor, a relatively high resistance in the collector circuit in thefirst transistor, and a relatively low resistance in the collectorcircuit of the second transistor, the bases of the first and secondtransistors individually constituting the first and second inputs of thedifferential amplifier and the collector of the first transistorconstituting the output of the differential amplifier.
 7. Apparatus asdefined in claim 1, in which the second DC voltage coupling meanscomprises third, fourth and fifth transistors connected in cascade. 8.Apparatus as defined in claim 7, further comprising means coupled to thecollector of the first transistor for reducing the second DC voltagewhen the motor is over-loaded.
 9. Apparatus as defined in claim 7, inwhich the base of the third transistor is coupled to the output of thedifferential amplifier and in which the collector emitter-circuit of thefifth transistor is connected in series with the DC-motor.
 10. In aspeed control system for a DC motor wherein the system comprises, incombination, a differential amplifier having first and second inputs,means responsive to the output of the differential amplifier forcoupling to the motor a first DC voltage proportional to the outputvoltage of the differential amplifier, means coupled to the motor forgenerating a second DC voltage proportional to the motor speed, meansfor generating a third DC voltage corresponding to a desired speed ofthe motor, means for coupling the second DC voltage to the first inputof the differential amplifier, means for coupling the third DC voltageto the second input of the differential amplifier, and stabilizing meansassociated with the system for maintaining the speed of the motorrelatively constant with changes in temperature, the improvementwherein: the third DC voltage generating means comprises, incombination, an emitter follower having adjustable output resistancemeans, and means including a single resistance bridge circuit having aninput connectable to a DC source and an output coupled to the input ofthe emitter follower, said bridge circuit including a Zener diode in onearm and an adjustable resistance in another arm for compensating thefinite resistance of the Zener diode, the output of the emitter followerconstituting the output of the third DC voltage generating means; andthe stabilizing means comprises temperature-dependent resistive meansinterconnectiNg the output of the second DC voltage generating means andthe first input of the differential amplifier, said resistive meansbeing individually associated with the last-mentioned generating meansand the differential amplifier for respectively compensatingtemperature-induced changes in the second DC voltage andtemperature-induced changes in the operating point of the differentialamplifier.
 11. Apparatus for regulating the speed of a DC motor, whichcomprises: means coupled to the motor for generating a first DC voltagenormally proportional to the motor speed, said generating means beingsusceptible to temperature-dependent changes in the first DC voltage; adifferential amplifier having first and second inputs and an output, theamplifier being susceptible to temperature-induced changes in itsoperating point; temperature-dependent resistive means interconnectingthe output of the first DC voltage generating means and the first inputof the differential amplifier, said resistive means being individuallyassociated with the last-mentioned generating means and with thedifferential amplifier for respectively compensating both thetemperature-induced changes in the first DC voltage and thetemperature-induced changes in the operating point of the differentialamplifier; means responsive to the output of the differential amplifierfor coupling to the motor a second DC voltage proportional to the outputof the differential amplifier, the second DC voltage coupling meanscomprising first, second and third transistors connected in cascade; avoltage divider having input terminals connected between the output andinput of the third transistor; means coupled to the output of thedifferential amplifier for reducing the second DC voltage when the motoris overloaded, the reducing means comprising a fourth transistor havinga collector connected to the input of the first transistor, an emitterconnected to the output of the third transistor and a base connected tothe output of the voltage divider; means for generating a stable motorspeed control voltage comprising, in combination, an emitter followerhaving adjustable output resistance means, and means including a singleresistance bridge circuit having an input connectable to a third DCvoltage and an output coupled to the input of the emitter follower, thebridge circuit including a Zener diode in one arm and an adjustableresistance in another arm for compensating the finite resistance of theZener diode, the output of the emitter follower constituting the outputof the control voltage generating means; and means for coupling theoutput voltage of the control voltage generating means to the secondinput of the differential amplifier.